1. Field of the Invention
The present invention relates to a composite insulator, a segment used for producing the composite insulator and a method of producing the composite insulator which can improve the composite insulator consisting of a core member and a housing member arranged around the core member, the housing member being constructed by a trunk portion and a shed portion.
2. Related Art Statement
Generally, various types of insulators have been known as insulation members for use as a main body of, for example, a bushing. FIG. 8 is a schematic view showing one embodiment of a known composite insulator. In FIG. 8, a composite insulator 51 comprises a core member 52 made of, for example, a FRP tube, and a housing member 55 arranged around the core member 52. Moreover, the housing member 55 is constructed by a trunk portion 53 and a plurality of shed portions 54 arranged outwardly from the trunk portion 53. Further, fitting members 56 are arranged at both end portions of the core member 52. In the known composite insulator 51 mentioned above, the housing member 55 comprising by the trunk portion 53 and the shed portion 54 is preferably made of a silicone rubber. The shed portion 54 has an upper plane 54a and a lower plane 54b. Here, the upper plane 54a and the lower plane 54b mean planes positioned at an upper side and at a lower side of the shed portion 54 with respect to the upright composite insulator 51.
The known composite insulator 51 having the construction mentioned above is produced by using a mold 61 as shown in FIG. 9. In FIG. 9, the mold 61 comprises a through-hole 62 through which the core member 52 made of the FRP tube is arranged, and a shed forming recess 63 for forming the shed portion 54. The mold 61 may be formed by an integral type. At first, the core member 52 is set in the through-hole 62 by means of the fitting members 56. Since an outer diameter of the core member 52 is smaller than an inner diameter of the through-hole 62, a space for forming the trunk portion 53 can be arranged between the core member 52 and the mold 61. Then, an elastic polymer material, preferably a silicone rubber, is supplied from an elastic polymer material inlet 65 into the space between the core member 52 and the mold 61. The supplied elastic polymer material is then cured by heating the mold 61. Finally, the mold 61 is moved for example downwardly to obtain the composite insulator 51. In this case, since the elastic polymer material after curing maintains its elasticity, the shed portion 54 is not damaged even if the mold 61 is forcedly pulled out from the composite insulator 51.
In the known composite insulator 51, a shape of the lower plane 54b of the shed portion 54 is plain. This is because, in a producing method of the known composite insulator 51, it is not easy to remove the mold 61 and also it is difficult to supply the silicone rubber into a tip of the shed portion 54, if the shed forming recess 63 of the mold 61 has a complicated shape. Therefore, since it is not possible to increase longer a creepage distance on the shed portion 54, there occurs a drawback that the composite insulator 51 can not be constructed in a compact manner. Moreover, in the case of using a thin shed portion 54, the composite insulator 51 is not damaged if the mold 61 is pulled out from the composite insulator 51. However, if the shed portion 54 is made thicker, it is difficult to remove the mold 61 from the composite insulator 51, and thus, in an extreme case, the shed portion 54 is fractured. Further, if the composite insulator 51 is produced by forming the shed portion 54 and the trunk portion 53 by means of a mechanical working without using the mold 61, it is difficult to work the lower plane 54b of the shed portion 54 into a complicated shape.